1. Field of the Invention
The present invention relates to systems and methods that are used to separate molecular hydrogen from a volume of gas. More particularly, the present invention is related to systems and methods that separate hydrogen from a volume of mixed gas by exposing the mixed gas to a hydrogen permeable material through which only atomic hydrogen can readily pass.
2. Prior Art Description
In industry, there are many applications for the use of molecular hydrogen. However, in many common processes that produce hydrogen, the hydrogen gas produced is not pure. Rather, when hydrogen is produced, the resultant gas is often contaminated with water vapor, hydrocarbons and/or other contaminants. In many instances, however, it is desired to have ultra pure hydrogen. In the art, ultra pure hydrogen is commonly considered to be hydrogen having purity levels of at least 99.999%. In order to achieve such purity levels, hydrogen gas must be actively separated from its contaminants.
In the prior art, one of the most common ways to purify contaminated hydrogen gas is to pass the gas through a membrane made of a hydrogen permeable material, such as palladium or a palladium alloy. As the contaminated hydrogen gas passes through the membrane, atomic hydrogen permeates through the walls of the membrane, thereby separating from the contaminants. In such prior art processes, the membrane is typically heated to at least three hundred degrees centigrade. Molecular hydrogen disassociates into atomic hydrogen on the surface of the membrane and the material of the membrane absorbs the atomic hydrogen. The atomic hydrogen permeates through the membrane from a high pressure side of the membrane to a low pressure side of the membrane. Once at the low pressure side of the membrane, the atomic hydrogen recombines to form molecular hydrogen. The molecular hydrogen that passes through the membrane can then be collected for use.
In certain types of hydrogen separators, the hydrogen permeable membrane is formed as a straight tube. For instance, in U.S. Pat. No. 5,997,594, to Edlund, entitled Steam Reformer With Internal Hydrogen Purification, a straight segment of palladium tubing is placed inside a larger tube. Gas is then caused to flow through the larger tube. Hydrogen from the gas permeates into the palladium tube, where it is collected.
The opposite configuration is shown in U.S. Pat. No. 6,461,408 to Buxbaum, entitled, Hydrogen Generator. In the Buxbaum design, a small diameter tube is placed inside a straight length of palladium tubing. Gas is introduced into the palladium tubing. Hydrogen from the gas permeates out of the palladium tubing and is collected. The remaining waste gas is removed by the small diameter tube.
In prior art systems like the ones shown in both the Edlund patent and the Buxbaum patent, gas is caused to flow either along the inside of a palladium tube or outside a palladium tube. However, in both prior art designs, the space though which the gas flows is large. This allows the gas to have a laminar flow as it passes along the length of the palladium tube. Due to the laminar flow characteristics of the passing gases, there is very little turbulence in the flowing gases. The laminar flow pattern prevents much gas from even contacting the surfaces of the palladium tube before the gases flow out of the palladium tubing. Accordingly, much of the hydrogen that may be contained in the flowing gas never has the opportunity to be absorbed by the palladium tubing. The hydrogen is merely flushed through the palladium tubing. The overall efficiency of the hydrogen separator therefore remains low.
Power & Energy, Inc., the assignee herein, has developed a line of hydrogen separators where a palladium alloy tube is positioned around a gas supply tube. The gap space in between the concentric tubes is very small. Consequently, gas flowing through the gap space flows in a turbulent fashion and results in more gas reacting with the hydrogen permeable membrane. Such hydrogen separators are exemplified By U.S. patent application Ser. No. 12/053,528, to Bossard, entitled Hydrogen Gas Separator System Having Micro-channel Construction For Efficiently Separating Hydrogen Gas From A Mixed Gas Source, and U.S. Pat. No. 7,704,485 to Bossard, entitled System And Method For Processing Fuel For Use By A Fuel Cell Using A Micro-Channel Catalytic Hydrogen Separator.
A problem that has occurred with such small gap hydrogen separators is that it has proven difficult to place catalyst material within the small gap that is available. Normally, catalyst material helps separate hydrogen from larger molecules within the supply gas. However, if the catalyst is not properly applied, it can actually hamper the gas flow rate through the gap space and decrease the efficiency of the overall hydrogen separator.
A need therefore exists for a system and method of applying a catalyst in the gap space between concentric tubes of a hydrogen separator. This need is met by the present invention as described and claimed below.